The present invention relates generally to data communication systems and more particularly relates to an apparatus for and method of acquiring synchronization in a spread spectrum communications transceiver.
The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming increasingly common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for amore reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communications systems were developed for use in military environments to overcome high energy narrowband enemy jamming. In commercial or home environments, they may be used to achieve reliable communication on noise media such as the AC power line, In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the power line. For example, electronic dimming devices can place large amounts of noise onto the power line since these devices typically employ triacs or silicon controlled rectifiers (SCRs) to control the AC waveform in implementing the dimming function.
A communication medium such as the AC power line may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
The spread spectrum receiver is required to perform synchronization that is commonly achieved using some form of acquisition method optionally in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC power line, the tracking loop typically fails frequently causing loss of information. Communication systems to overcome these problems are large, complex and expensive.
Synchronization of signals between a transmitter and receiver that are communicating with each other in a spread spectrum communication system is an important aspect of the process of transmitting signals between them. Synchronization between the transmitter and the receiver is necessary to allow the despreading of the received signals using a spreading code that is synchronized between them so that the originally transmitted signal can be recovered from the received signal. Synchronization is achieved when the received signal is accurately timed in both its spreading code pattern position and its rate of chip generation with respect to the receiver""s spreading code.
One of the problems associated with synchronization is that the techniques used to synchronize two signals are relatively expensive to implement. In communication systems having sophisticated and relatively expensive central communication sites which serve a plurality of relatively inexpensive remote communication sites, it is desirable to reduce the cost of synchronization systems in the remote communication sites while not increasing the cost of the central communication sites.
In a communications transceiver, it is desirable that the acquisition mechanism be more reliable that any error correction code used for the data portion of the packet. In other words, it is preferable to declare synchronization correctly and not be able to correctly decode the packet data than to miss the entire packet altogether because of a weak acquisition algorithm.
Further, it is desirable that the acquisition algorithm has as low a probability as possible of false synchronization from noise, e.g., less than once in 5 seconds. The acquisition mechanism should be capable of utilizing more than one synchronization sequence whereby the probability of synchronization from another sequence is minimized.
Accordingly, the present invention provides a novel and useful acquisition and synchronization mechanism. The mechanism of the present invention is useful in communication systems characterized by shared media such as networks that use power line carrier communications. In general, the invention is applicable where a plurality of stations are connected to a shared communication media whereby receiving stations must acquire synchronization on a start of packet signal transmitted by transmitting stations at the beginning of each packet.
An improved acquisition mechanism for spread spectrum communication systems is provided whereby a synchronization sequence comprising a plurality of known symbols spaced apart by predefined time delay intervals is transmitted as the start of packet signal. At the receiver, the received signal is correlated against the synchronization sequence using the predefined gaps or time delay intervals inserted between the symbols.
The received signal is first passed through a linear correlator which functions to generate a correlation peak for each symbol received. The expected position of each correlation peak is then calculated and compared to the positions of the correlation peaks received. If the number of matches exceeds a threshold, synchronization is declared.
The acquisition algorithm is adapted to search for matching correlation peaks while considering zero or more received symbols in error. Further, the algorithm permits a match if the expected correlation position is within a predefined delta of the received correlation peak. If unsuccessful, the acquisition algorithm repeats in an attempt to correlate each predefined synchronization sequence to the received correlation peaks.
Once synchronization is declared, a synchronization quality factor is calculated as a function of the number of matches and the number of correlation peaks whose value exceeds a threshold. If subsequent synchronizations are declared, the quality factors are compared and if the latest quality factor is greater, the previous packet is dropped and the current packet is received. Note that the process of comparing synchronization quality and dropping the previous packet in favor of the next packet is performed until the header CRC checksum field is verified. After the header CRC is checked and verified, the receiver is locked into receiving the current packet.
The acquisition mechanism provides for multiple synchronization sequences wherein the cross correlation of the sequences is minimized in order to reduce the probability of false detection with another sequence. The use of multiple synchronization sequences permits additional information to be transmitted to the receiving station. For example, the different sequences may be adapted to indicate to the receiver the particular packet type or modulation scheme used for that packet transmission.
Many aspects of the previously described invention may be constructed as software objects that execute in embedded devices as firmware, software objects that execute as part of a software application on a computer system running an operating system such as Windows, UNIX, LINUX, etc., an Application Specific Integrated Circuit (ASIC) or functionally equivalent discrete hardware components.
There is thus provided in accordance with the present invention a method of acquiring synchronization on a start of packet signal comprising a plurality of symbols generated in accordance with a predefined synchronization sequence template, the method comprising the steps of correlating a receive signal to generate a plurality of received correlation peaks, calculating an expected position of each received correlation peak in accordance with the predefined synchronization sequence template, determining a correlation quality at the expected positions and evaluating the correlation qualities and declaring synchronization if the evaluation exceeds predetermined criteria.
There is also provided in accordance with the present invention a method of generating a start of packet synchronization sequence, the method comprising the steps of generating a plurality of N symbols to be transmitted in the synchronization sequence, generating Nxe2x88x921 predetermined signals, inserting one of the Nxe2x88x921 predetermined signals after each of the first Nxe2x88x921 symbols in the synchronization sequence and wherein N is a positive integer.
There is further provided in accordance with the present invention an apparatus for acquiring synchronization in a communications network on a start of packet synchronization sequence comprising a plurality of symbols, each pair of symbols spaced apart in accordance with a predetermined synchronization template comprising a correlator adapted to generate a correlation peak in response to each received symbol in the synchronization sequence, an acquisition circuit comprising means for calculating an expected position of each received correlation peak in accordance with the predefined synchronization template, means for determining a correlation quality at the expected positions and means for evaluating the correlation qualities and declaring synchronization if the evaluation exceeds predetermined criteria.
There is also provided in accordance with the present invention a method of acquiring synchronization in a communications network, the method comprising the steps of receiving a synchronization sequence at the beginning of packet sent by a transmitting station, the transmitted synchronization sequence comprising a plurality of symbols wherein each symbol is separated by a predetermined signal, correlating the received synchronization sequence to generate a plurality of correlation peaks, searching for the presence of correlation peaks in expected positions within a predefined delta wherein the expected positions correspond to the predetermined time delays inserted between symbols by the transmitting station, declaring synchronization if the number of correlation peaks found in corresponding expected positions within the delta exceeds a threshold and determining a synchronization time point as a function of the distances between the positions of the received correlation peaks and their corresponding expected positions.
There is further provided in accordance with the present invention an Application Specific Integrated Circuit (ASIC) for acquiring synchronization in a communications network on a transmission signal including a start of packet synchronization sequence of a plurality of symbols, each pair of symbols with a predetermined gap inserted therebetween in accordance with a predetermined synchronization sequence gap template, the ASIC comprising receiving means adapted to generate a receive signal from the transmission signal, correlator means adapted to generate a correlation peak from the receive signal in response to each received symbol in the synchronization sequence, calculating means for calculating an expected position of each received correlation peak in accordance with the predefined synchronization gap template, determining means for generating a correlation quality at the expected positions and means for evaluating the correlation qualities and declaring synchronization if the evaluation exceeds predetermined criteria.
There is also provided in accordance with the present invention a communications station for transmitting and receiving signals to and from other stations connected over a shared communications media based network comprising a coupling circuit for generating a receive signal received over the network and for outputting a transmit signal onto the network, a transmitter adapted to modulate a synchronization sequence and data to be transmitted in accordance with a modulation scheme so as to generate the transmit signal therefrom, the synchronization sequence comprising a plurality of symbols wherein each symbol is separated by a first signal in accordance with a predetermined synchronization sequence template, a receiver adapted to demodulate the receive signal in accordance with the modulation scheme so as to generate a receive data signal therefrom, the receiver comprising an acquisition circuit comprising, means for correlating the receive signal to generate a plurality of received correlation peaks therefrom, means for calculating an expected position of each received correlation peak in accordance with the predetermined synchronization sequence template, means for comparing the position of each received correlation peak with a corresponding expected position and declaring a match if the position of a received correlation peak is within a predetermined distance of the corresponding expected position, means for declaring synchronization if the number of matches exceeds a threshold, a media access control (MAC) circuit adapted to interface an application processor to the shared communications media and the application processor adapted to control the operation of the transmitter, receiver and MAC and to provide an interface between the MAC and an external host.